The invention concerns the production of multifocal correction lenses, in particular optical lenses for human-vision improving glasses when the necessary correction varies depending upon the distance of the observed object. Such is the case for presbyopia which, as is well known, mainly leads to lenses having a double or triple focus (so-called bifocal or trifocal lenses), or to lenses wherein the focal distance progressively varies from one point of the lens to another (commonly called progressive lenses). The invention in particularly concerns a method for producing such lenses, as well as a system for implementing such a method, i. e. in particular an automated system for producing correction lenses, controlled by a data processing system with a recorded program.
In the prior art, it is interesting to recall that the U.S. Pat. No. 2,310,925 discloses lenses of the bifocal (for distant-vision and near-vision respectively) or trifocal type, U.S. Pat. No. 2,869,455 discloses the invention of progressive lens and U.S. Pat. No. 5,430,504 describes a production technology for the so-called merged lens wherein the jump between two zones with different focuses is dimmed. These documents widely explain the method for producing multifocal lenses and the machining performed on the front face, or convex external face. Since the curvature radius of the concave face generally is uniform, this convex face is the surface on which the different curvature radii selected on the basis of wished powers are introduced, implying lens thickness variations. It is then assumed that the lens everywhere consists of the same transparent, mineral or organic material.
Those lenses often are blamed for their unaesthetic aspect, resulting from strong thickness variations. Another group of methods dispenses from using a single material having the same reflection index in each zone of the optical lens. Such methods then provide for two materials with different reflection indices, whereby an auxiliary small-diameter lens is incorporated, by fusion, into the material of the main, large-diameter lens. This incorporation again is performed on the front face of the main lens. The main lens is designed for distant-vision correction and the auxiliary lens has a complementary correction for near-vision correction. Both corrections essentially are obtained by the relative value of the refraction indices, without requiring any difference of the curvature radii. The variation of the global power is easily made progressive, from one point of the lens to another, by varying the thickness of the layers having different indices.
These lenses however are not free from inconveniences. In particular, passing from the distant-vision to the near-vision causes image jumps that are troublesome, and unavoidable, for the user. In an attempt to attenuate this type of inconvenience, trifocal lenses can be preferred, but again at the expense of the aesthetic aspect, due to sensible thickness variations. The typical correction ranges, in focal distances, extend from 0,3 to 0,5 m for near-vision, from 0,5 to 1 m for intermediate vision and from 2 m to infinity for distant-vision.
The present industrial conditions in practice imply fabricating semi-finished lenses with various usual corrections which the industry makes available to the opticians, so that the latter only will have to adapt the positioning of the main curvature center for each person. Those conditions furthermore tend to favor the aesthetic aspect since they resort to an index variation (plus potentially a progressive power variation by surfacing) rather than a thickness variation, without taking into account the fact that the angular deviation between the view orientations in distant-vision and near-vision varies from one person to the next. More generally, nothing is done to insure an optimal user comfort.
The main object of the present invention consequently is an improvement of the visual comfort adapted to each person, without however neglecting the aesthetic aspect. An additional object is the respect of the optimal conditions for industrial feasibility, in particular by starting from semi-finished lenses such as those that presently are available, and operating easy to use, low cost equipment.
To meet these objects in a method for producing vision correction lenses wherein near-vision correction results from a power addition as compared to distant-vision correction, the invention essentially proposes performing on an internal face of each lens a mechanical machining that adds a prismatic deviation, with a reduction of the lens thickness, which prismatic deviation is calculated, based on an individual distance between a distant-vision application center and a near-vision application center, in order to bring back the optical correction center in near-vision as close as possible to the near-vision application center.
In practice, the method implementation advantageously starts from the semi-finished lenses in which said addition is realized, at least for a major part, by varying the reflection index of the transparent material making up said lens at the level of an external face of the lens, and the complementary machining of the invention is then performed by surfacing the opposite internal face.
For the most common multifocal lenses, with preferably progressive power variation, the prismatic variation to be performed varies between 0,5 and 1,5 dioptries. In the simplest case, it is applied as a single correction, centered along the view motion axis between distant-vision and near-vision. It then is very easily obtained by interposing a properly sized wedge, between the semi-finished lens and its support, in order to bring out of center the machining axis, by spherically milling the internal face of the lens.
To further improve the operating conditions and the industrial practice of the invention, it often is advantageous to admit that the deviation data to be taken into account, between the distant-vision and near-vision centers, is the same for all individuals who have the same addition value for near-vision correction, for a determined distant-vision correction value.
In a preferred embodiment for industrial applications, the invention particularly concerns a method for producing, in particular for presbyopia glasses, a multifocal correction lens from a semi-finished lens with determined optical characteristics, whereas said semi finished lens comprises a first concave face and a second convex face and includes at least a first positioning marker M associated with a so-called distant-vision correction A, and a second positioning marker Mxe2x80x2, associated with a so-called near-vision additive correction B, both located on said convex face and consisting of points, said method being characterized in that it at least includes a surfacing step wherein material is removed at a determined depth from one of said faces by means of abrasion machining means being translated along a first axis, in that said surfacing step includes presenting said semi-finished lens so that it faces said machining means and that a second axis orthogonal to a tangent plane at the point constituting said first positioning marker M is inclined at a determined angle with respect to said first axis, so as to induce into the semi-finished lens a prism aligned on said rectilinear segment {overscore (MM)}xe2x80x2 and having an apex angle which is a function of said inclination angle, and in that the prismatic deviation xcex94xe2x80x2, in dioptries, of said induced prism complies with the relation:
xcex94xe2x80x2=({overscore (MM)}xe2x80x2xc3x97A)+Yxc3x97(A+B)
where
{overscore (MM)}xe2x80x2 is the distance in centimeters between said points M and Mxe2x80x2, A and B are said corrections expressed in dioptries, and y is the distance in centimeters between the point Mxe2x80x2 and the optical near-vision center of said correction lens.